FIG. 1 is a schematic view of a displacement measuring interferometer according to the prior art. The displacement measuring interferometer of FIG. 1 principally comprises a laser source 10, a beam splitter 11, a reference surface 12, an object 13 and optical detector 14. The laser source 10 is a single wavelength coherent light source for emitting a laser beam. By the beam splitter 11, the laser beam emitted by the laser source 10 is split into two orthogonal polarized beams, i.e. a first sub-beam and a second sub-beam. The first sub-beam is directed to the reference surface 12, and the second sub-beam is directed to the object 13. The first sub-beam and the second sub-beam respectively reflected by the reference surface 12 and the object 13 are directed to the optical detector 14 through the beam splitter 11. On the optical detector 14, the first sub-beam interferes with the second sub-beam to produce an interference pattern relating to the wavelengths of the laser beams. According to a change of the interference pattern on the optical detector 14, a displacement between the object 13 and a predetermined location is deduced.
FIG. 2 is a schematic view of another displacement measuring interferometer according to the prior art. The displacement measuring interferometer of FIG. 2 principally comprises a laser source 20, a polarization beam splitter 21, a reference surface 22, a first quarter wave plate 220, an object 23, a second quarter wave plate 230, an analyzer 24 and an optical detector 25. The laser source 20 can emit a laser beam that is a combination of a P-polarized light of a first wavelength (or frequency) and an S-polarized light of a second wavelength (or frequency). By the polarization beam splitter 21, the laser beam emitted by the laser source 20 is split into two orthogonal polarized beams, i.e. a first sub-beam and a second sub-beam. According to a special design, the P-polarized light is reflected by the polarization beam splitter 21 but the S-polarized light is penetrated through the polarization beam splitter 21. As such, the P-polarized first sub-beam is directed to the reference surface 22 through the first quarter wave plate 220, reflected by the reference surface 22, and changed into an S-polarized first sub-beam after passing through the first quarter wave plate 220. Whereas, the S-polarized second sub-beam is directed to the object 23 through the second quarter wave plate 230, reflected by the object 23, and changed into a P-polarized second sub-beam after passing through the second quarter wave plate 230. The S-polarized first beam and the P-polarized second sub-beam are returned to the polarization beam splitter 21. Due to the optical characteristic properties, the S-polarized first beam is penetrated through the polarization beam splitter 21 but the P-polarized second sub-beam is reflected by the polarization beam splitter 21. Under this circumstance, the S-polarized first beam and the P-polarized second sub-beam are directed to the same side of the polarization beam splitter 21. After the S-polarized first beam and the P-polarized second sub-beam are directed through an analyzer 24, a reference signal associated with a frequency difference (f1−f2) between a first frequency f1 and a second frequency f2 is generated on the optical detector 25. Since the frequency of interference signal is influenced by the Doppler effect resulting from the moving object 23, a displacement between the object 23 and a predetermined location 26 is deduced.
The above-mentioned displacement measuring interferometers can be used to precise positioning machines such as semiconductor machines or precise machining apparatuses. The conventional displacement measuring interferometers, however, still have some drawbacks. For example, the tilting angle of the object fails to be measured by the conventional displacement measuring interferometers. In addition, the tilting angle of the object may adversely influence the accuracy of the measured displacement.
Therefore, there is a need of providing a tilting angle measuring device to obviate the drawbacks encountered from the prior art.